Broadband cornucopia-type antenna system

ABSTRACT

The present invention describes a broadband, low-profile circularly polarized antenna, primarily for airborne applications which has a substantially omnidirectional radiation characteristic in the horizontal plane. The form factor of the antenna comprises a cornucopia-shaped element having an arcuate surface which is defined by a plurality of polynomial line segments and which diverges from one terminal end to the other.

=1 W N Y [72] Inventor Lawrence V. Belnr Greenville, NC. [21] Appl. No. 708,252 [22] Filed Feb.26,1968 [45] Patented Nov. 2,1971 [73] Assignee Multronics lnc.

Roclrville, Md.

[54] BROADBAND CORNUCOPlA-TYPIE ANTENNA SYSTEM 5 Claims, 12 Drawing Figs.

[52] US. Cl 343/745, 343/845, 343/848, 343/895 51 llnt.Cl new 9/00, H01ql/36,H01q H48 [50] Field 01 Search 343/74l-748, 708, 847-848, 829-830, 843, 895

[56] References Cited UNITED STATES PATENTS 2,492,404 12/1949 Streib ct al 343/791 X 2,615,134 10/1952 Carter 343/848 X 3,015,101 l2/l96l Turner et al. 343/895 X 3,087,159 4/1963 G0zinsky.... 343/895 X 3,146,454 8/1964 Caron..... 343/895 X 3,208,072 9/1965 Miley 343/895 3,366,963 l/l968 Goff 343/895X FOREIGN PATENTS 704,659 2/1954 Great Britain 343/746 OTHER REFERENCES Picken, Jr. et al., Remote Mobile-Antenna Resonating, QST, 12- l953,pp. 34

Wolff, E. A., Antenna Analysis," John Wiley & Sons, 1966, Tl(7872 A6W56 pp. 106-108 Blake, L. N., Antennas," John Wiley & Sons, 1966, pp. 80 83 Very High-Frequency Techniques, Vol. 1 H. J. Reich, Editor Radio Research Laboratory Harvard U., McGraw Hill, 1947, pp. 2- 3 Primary Examiner-l lerman Karl Saalbach Assistant Examiner-William l-l. Punter Attorney-Brady, OBoyle and Gates having an arcuate surface which is defined by a plurality of polynomial line segments and which diverges from one terminal end to the other.

PATENIEUmwz asn SHEET 1 [1F 3 E H 8 M E V W C N E R W A L ATTORNEYS PMENTEURM m1 ameum SHEET 2 BF 3 i a IIE E." Ill BQSlBJOd PATENTED HUVZ um SHEET 3 OF 3 11 BROADBAND CORN UCOPIA-TYPE ANTENNA SYSTEM BACKGROUND OF THE INVENTION The present invention pertains to broadband antennas which are relatively frequency independent. Furthermore, it is directed to antennas having a folded-over shorted base, wherein its broad end is electrically and mechanically attached to the ground plane. Such antennas are particularly suited for use on aircraft where there is a requirement for simplicity, ruggedness and ease ofmounting.

Prior art apparatus of the type described is known to those skilled in the art. For example, US. Pat. No. 3,015,101, issued to E. M. Turner et al., describes a coplanar equiangular stub antenna having a folded over shorted base and is referred to as a scimitar antenna. The inner and outer boundaries follow logarithmic spiral paths, the general equation of which is, p=k" where p is equal to the distance from the origin to the curve, a is the direction of p measured from the starting point, e is the natural logarithm base, and k and a are constants which provide the parameters through which varied forms of the antenna elements may be derived. The feed point is substantially coplanar with the ground plane.

SUMMARY OF THE INVENTION The present invention is directed to a broadband antenna system comprising a cornucopia or Horn of Plenty-shaped antenna element having an arcuate surface which diverges from one terminal end including the feed point to the other end which is terminated in the ground plane. The feed point is situated a predetermined distance above the ground plane so that it is not coplanar therewith. In addition, the cross section of the antenna element can be selectively either polygonal or circular but preferably rectangular. The present invention also contemplates remote tuning means coupled to the antenna element comprising a terminating transmission line and a variable capacitor driven by motor means operable to vary the capacitor setting between two limit stops.

DESCRIPTION OF THE DRAWINGS FIG. I is a profile view of one embodiment of the subject invention including a tuning matrix coupled to the feed point;

FIG. 2 is a graph helpful in understanding the subject inventron;

FIG. 3 is a profile view of another embodiment of the subject invention;

FIG. 4 is a view shown partly in cross section of the feed point connection utilized by the subject invention;

FIG. 5 is a perspective view of still another embodiment of the subject invention;

FIG. 6 is a profile view of an antenna similar to the embodiment shown in FIG. 11 including a preferred embodiment of the tuning means utilized by the subject invention;

FIG. 7 is a plan view of the configuration shown in FIG. 6;

FIG. 8 is a cross-sectional view of the configuration shown in FIG. 7 taken along the line 88;

FIG. 9 is an enlarged cutaway view ofa portion of the configuration shown in FIG. 6 illustrating in detail the means for remotely controlling the tuning of the antenna element; and

FIG. 10 is a cross sectional view taken along the lines 9-9 ofthe mechanical arrangement shown in FIG. 9;

FIG. 11 is a fragmentary profile view of another means for remotely controlling the tuning of the antenna element; and

FIG. 12 is an electrical diagram of the input tuning element shown in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, attention is directed to FIG. 1 wherein the first and preferred embodiment of the subject invention comprises an antenna element 10 having its smaller end termination 12 coupled to a tuning matrix M including a feed point connector 40, not shown, while the larger end termination 116 is mechanically attached to a ground plane 18 by means of the flanges 20 and the hardware 22. The antenna element 10 has a rectangular cross section such that the upper surface 24 and the lower surface 26 comprise upper and lower walls of electrically conductive material such as copper sheeting or the like. The near sidewall 28 is also comprised of the same material so that the four walls define a cornucopiashaped antenna element referred to hereinafter as the "Horn of Plenty or I-IOP antenna. The upper and lower surfaces 24 and 26, respectively, have an arcuate nonlogarithmic surface which diverges from the small end termination 12 to the larger end termination 16 attached to the ground plane 18. In addition to the mechanical contact, an electrical connection is made to the ground plane 18 by means of the large terminal end 16. The sidewalls of sidewall 28 are not parallel but flare out from the feed point end termination 12 to the ground plane 18. This is illustrated by referring to FIG. 7 which illustrates sidewalls 28 and 30 flaring out in a straight line from the termination 12. This embodiment provides a substantially rectangular cross section for the HOP antenna element 10 comprised of a lateral closed surface formed by the mentioned walls which forms a nonlogarithmic arcuate-shaped closed surface about a nonlogarithmic arcuate central axis.

Referring again now to FIG. 1, the contour of the upper and lower surfaces 24 and 26 diverge from the feed point end 112 in an arcuate path to the ground plane 18. The feed point termination 12 of the HOP antenna 110 is located a predetermined distance above the ground plane 18 as opposed to the scimitar antenna whose upper and lower surfaces are defined by a logarithmic spiral whose origin originates in the ground plane. The surfaces 24 and 26 are defined as having a cornucopia-shaped surface or a nonlogarithmic arcuate closed surface which can be more particularly described as comprising a plurality of polynomial line segments of the general form:

In greater detail, the contour of the surfaces 24 and 26, respectively, can be more fully understood by referring to FIG. 2 wherein a graph is illustrated comprising curves a and b corresponding to the upper and lower surfaces 24 and 26, respectively, of FIG. 1. Both curves a and b are divided into three line segments, for example considering curve a, 0 to x x to x and x to 1:

Applying the general form of the equation (I) to curves 0 and b of FIG. 2, the curve segments can be defined by the following mathematical expressions:

For curve a,

(6) where x x x,'

ject invention was taken and graph was made of the profile shown in FIG. 1. A polynomial fit was preformed by a digital computer for three well-defined segments from which the coefficients a a ,a and 11., were determined.

for y,=9 in. and x =23.13 in.

Curve a is described as follows:

for 0 x, g 7

a a a and a in equation (2) became for 7 x 14 11 ,11 ,11 and a in equation (3) became for 14 5 x 5' 23.!3

a,, a a and a in equation (4) became Curve b is described as follows:

forx =5.l3 and 5.13 x

a 11,, a a in equation (5 became a 22.072,839 (l I) for 10 x 5 .16

a a,, a and a in equation (6) became for 16 g x, g 23 a,, a a and a in equation (7) became The empirical curves were derived from measurements on a HOP antenna with x -23 inches. However it is not difficult to 0 use these curves to generate HOP antennas with different values of x For an arbitrary value of x, let

Next, compute all the y(l) using the empirical formulas.

Finally, multiply all the v(t) by x /23 40 Referring back to FIG. 1, the embodiments shown therein additionally include a coaxial connector 32 which is coupled to the tuning matrix 14 by means of a circuit lead 34. The coaxial connector 32 is adapted to receive the RF signal to be radiated from the HOP antenna element 10 and is coupled to the feed point termination 12 by means of the tuning matrix 14 the purpose of which is to vary the input capacity of the antenna at the feed point.

Electrically the antenna radiates a substantially omnidirectional pattern in the horizontal plane and has a relatively broad bandwidth as well as the essentially circular polarization characteristic. The VSWR does not exceed 2.0:] over the specified frequency range. In some instances, the VSWR may be as low as 1.5:1 at a specified carrier frequency.

It has been observed that the current distribution along the walls of the HOP antenna element 10 is primarily concentrated in the upper segments which might correspond to the segments x, through x of curve a and x through an, ofcurve b as shown in FIG. 2. When desirable, therefore, the antenna element 10 may be truncated into a configuration as shown in 60 FIG. 3 where a HOP antenna element 36 includes upper and lower surfaces 24 and 26, respectively; however, a substantially planar frontal surface 38 extends from a predetermined point on the upper surface 24 to the ground plane 18 where it meets the lower surface 26 in the flange 20. The feed point 65 end termination of the antenna element 36 is identical to the embodiment shown in FIG. 1 and therefore is designated by the reference numeral 12. A connector element 40 is coupled to the feed point end termination 12 for the coupling of RF energy thereto. The mechanical details of the connector 40 70 are shown in FIG. 4.

Referring now to FIG. 4, there is illustrated in partial cross section a tubular male member 41 of the connector 40 which is adapted to mate with the finger elements 42 and the inner surface 44 of the housing 46 constructed preferably of an elec- 75 trically insulating material. The male member 41 is secured to the housing 46 by a press fit maintained by the spring action of the finger elements 42. The electrical energy to be radiated from the HOP antenna element is coupled to the fingers 42 whereupon it is fed to the antenna by means of the male tubular member 41.

The present invention additionally contemplates an antenna configuration which is substantially curvilinear in cross section as opposed to the embodiments shown in FIGS. 1 and 3 where a rectangular cross section is contemplated. Such an embodiment is shown in FIG. 5 where a cornucopia-shaped antenna element 48 having a substantially circular cross section is illustrated. In the same manner as the other embodiments described, the antenna horn element 48 includes a feedpoint end termination 50 and a broad flared end termination 52 which is adapted to be mechanically and electrically attached to the ground plane by means of the flanges 54. A tubular connector element is attached to the feed point end termination in the same manner as is disclosed with respect to FIG. 3 and illustrated in detail in FIG. 4.

Considering now FIGS. 6 and 7 simultaneously, there is shown a profile view and a plan view, respectively, of an antenna element 10 similar to the one shown in FIG. 1 with the exception that the feed point end termination 12 is slightly modified. What is additionally disclosed in FIGS. 6 and 7 is means for remotely tuning the HOP antenna element 10 for a predetermined operating or carrier frequency. The means contemplated comprises a sealed variable capacitor 50 having one terminal 51 electrically connected to the sidewall 28 intermediate the terminations l2 and 16 by means of the metallic strap 52 secured thereto by means of the flange 54 and hardware 56. This is shown in detail in the cross-sectional view shown in FIG. 8. The opposite terminal 58 of the capacitor 50 is connected by means ofthe metallic strap 62 to a terminating transmission line comprising in the present embodiment a stubbed coaxial cable 60. The coaxial cable 60 is not connected to the feed point termination 12 but operates as a tuning stub in series with a capacitor 50 which is electrically connected to the antenna by means of the strap 52. Movement of the electrical connection made by the metallic strap 52 along the sidewall 28 at the junction of the lower wall 26 is capable of changing the tuning of the antenna as it acts to change the capacitive distribution of the tuning means comprising the capacitor 50 and the stubbed coaxial cable 60. The variable capacity 50, moreover, is adapted to be varied by means of an electrical motor, not shown, having its output shaft, not shown, coupled to the shaft 64 of the capacitor by means of the coupling 66.

However, referring to FIG. 9, there is disclosed a cutaway portion of the broad end of the HOP antenna element 10 including means located internally thereof for turning the shaft 64 of the variable capacitor 50 by means of the mechanical coupling 66. The means comprises a motor assembly including a drive motor 68 mounted on a plate 70 which is attached to the base comprising the ground plane [8. The output shaft 72 is connected to a mechanical coupling 66 for turning the shaft 64. Mounted on the shaft 72 is one bevel gear 74 of a pair of bevel gears including bevel gear 76. Both bevel gears 74 and 76 are held in engagement by means ofa mounting bracket 78 also through which passes the shaft 72. Coupled to the bevel gear 76 is a threaded shaft acting as a lead screw 80 which is adapted to turn in accordance with the rotation of the shaft 72. A pair of notched screw nuts 82 and 84 are placed in engagement with the threaded shaft 80 and are adapted to be advanced or retarded along the shaft in accordance with the rotation of the motor shaft 72. A detent element 86 is mounted on the plate 70 so that it is in engagement with one of the notches of each of the screws 82 and 84 so that the screws will move linearly along the lead screw 80 as it rotates. A pair of electrical limit switches 88 and 90 are adapted to be actuated by the nuts 82 and 84, respectively, by contacting the fingers 92 and 94. The limit switches 88 and 90 are electrically connected to the drive motor 68 by electrical circuitry, not

shown, for shutting off the motor when the shaft 72 has driven the variable capacitor 50 between two predetermined limit stops.

The threaded shaft is adapted to feed through a bearing 96 to a universal coupling 98. The universal coupling 98 is connected to the shaft 100 of a selsyn device 102 which is adapted to drive a digital position indicator at a remote location not shown.

What has been achieved with the use of the apparatus as shown in FIG. 9 is a means for remotely tuning the variable capacitor 50 by actuating the drive motor 68 between two limit stops as controlled by the microswitches 80 and 90 being actuated by the sliding nuts 82 and 041, respectively, on the threaded shaft 80. Additionally, the selsyn device 102 is adapted to provide an indication of the setting of the variable capacitor 50 by driving a digital position indicator, not shown. By calibrating the setting of the variable capacitor 50 for a particular operating frequency, it is possible to tune the antenna for a particular operating frequency by operating the drive motor 68 between the prescribed limit stops to a predetermined indicator reading.

PK). is merely included for additional illustration of the configuration including the detent 86, the slidable nuts 82, and the threaded shaft 80. FllG. 10 is merely a cross section of the configuration taken along lines 9-9 of FIG. 9.

Referring now to MG. 11, a HOP antenna element 104, which is similar to the antenna element 110 shown in FIG. 6, includes another embodiment of a tuning arrangement therefor. The means utilized includes an insulated post 106 over which the tubular feed point member 10% is fitted. An input terminal assembly 110 is fitted over the other end of the insulated post 106 so that a separation exists relative to the tubular member 108.

A variable length transmission line element 112 is mounted near the feed point end termination of the HOP antenna element 10d and is comprised of two concentrically wound or bifilar helical coils 1114 and 116 with a movable shunt 113 which is adapted to ride along the inner surface of the coils in a helical spiral path. One end of the coil 1141 is connected to the input assembly 110 while the corresponding end ofthe coil 116 is connected to the tubular feed point member 08.

The length of the transmission line 112 is determined by the location of the movable shunt 118 which is driven by means of a suitable motor assembly, not shown, located in the housing 120. A shaft 122 couples the shunt to the motor assembly which may be similar to the embodiment shown in FIG. 9 but removed from inside of the antenna element 10. A second shaft 124 couples the motor assembly to a variable shunt capacitor 126 in a manner similar to the capacitor 50 shown in FIGS. 6 and 9. One end of capacitor 126 is coupled to the HOP antenna 104 by means ofa metal strap 128 while the op posite end of capacitor 126 is connected to the ground plane 18.

FIG. 12 is an electrical diagram of the variable length transmission line 112 coupled into the feed point circuit of the embodiment shown in H6. ill and is helpful in understanding the operation thereof. The purpose of this configuration is to provide more convenient tuning of the antenna system by remote control of the motor assembly in the housing 120. Also faster turning is achieved as well as greater control over the bandwidth.

What has been shown and described therefore is a broadband, low-profile antenna having essentially omnidirectional radiation and circularly polarized radiation characteristics as well as high power handling capability. The power handling capability of the antenna structure of the subject invention is limited only by the corona breakdown limitations of the antenna structure itself. Typically, an antenna structure as described by the subject invention is capable of handling well in excess of 100,000 watts of power. Since there are no dielectric plates or other lossy components in the antenna element itself, it is obvious that the majority of power put into the antenna of the subject invention is expended in radiation and is therefore an intrinsically extremely efficient antenna.

While there has been shown and described what is at present considered to be the preferred embodiments of the invention, modifications thereto will occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described but it is to be understood that all equivalents, alterations and modifications within the spirit and scope of the invention are herein meant to be included.

Iclaim:

1. A broadband antenna for electromagnetic radiations comprising an antenna element having an arcuate central axis, a lateral closed surface about said axis, a feed point termination located at one end of said closed surface and situated a predetermined distance above a conductive ground plane, an end termination located at the opposite end of said closed surface and connected to said ground plane, said lateral closed surface being diverged outwardly from said feed point termination to said end termination, means for coupling said feed point termination to a source ofelectromagnetic energy, and a terminating transmission line and capacitance means coupled together in series to said lateral closed surface intermediate said feed point end termination and said end termination for tuning said antenna element to a predetermined operating frequency.

2. The invention as defined by claim 1 wherein said capacitance means comprises a variable capacitor and a means coupled to said variable capacitor for effecting a predetermined capacitance value for a selected operating frequency.

3. The invention as defined by claim 2 wherein said means for effecting a predetermined capacitance value comprises an electrical motor having a shaft coupled to said variable capacitor and additionally including means for controlling said motor means to effect a capacitor setting between two limit stops.

4. The invention as defined by claim 13 wherein said means for controlling said motor means comprises a pair ofmechanically actuated switches located a predetermined distance away from one another, a threaded shaft coupled to said motor means and driven thereby, a pair of notched nuts threadably engaged on said threaded shaft and adapted to be respectively moved into contact with said pair of switches for actuation of said switches to effect a first and a second limit stop, and a detent element located adjacent said pair of notched nuts and being in mechanical engagement therewith for preventing rotation thereof so as to effect a linear move ment along said threaded shaft when said shaft is driven by said motor.

5. The apparatus as defined by claim 3 and additionally in cluding selsyn motor means coupled to said threaded shaft for providing an electrical readout of the mechanical setting of said variable capacitor. 

1. A broadband antennA for electromagnetic radiations comprising an antenna element having an arcuate central axis, a lateral closed surface about said axis, a feed point termination located at one end of said closed surface and situated a predetermined distance above a conductive ground plane, an end termination located at the opposite end of said closed surface and connected to said ground plane, said lateral closed surface being diverged outwardly from said feed point termination to said end termination, means for coupling said feed point termination to a source of electromagnetic energy, and a terminating transmission line and capacitance means coupled together in series to said lateral closed surface intermediate said feed point end termination and said end termination for tuning said antenna element to a predetermined operating frequency.
 2. The invention as defined by claim 1 wherein said capacitance means comprises a variable capacitor and a means coupled to said variable capacitor for effecting a predetermined capacitance value for a selected operating frequency.
 3. The invention as defined by claim 2 wherein said means for effecting a predetermined capacitance value comprises an electrical motor having a shaft coupled to said variable capacitor and additionally including means for controlling said motor means to effect a capacitor setting between two limit stops.
 4. The invention as defined by claim 13 wherein said means for controlling said motor means comprises a pair of mechanically actuated switches located a predetermined distance away from one another, a threaded shaft coupled to said motor means and driven thereby, a pair of notched nuts threadably engaged on said threaded shaft and adapted to be respectively moved into contact with said pair of switches for actuation of said switches to effect a first and a second limit stop, and a detent element located adjacent said pair of notched nuts and being in mechanical engagement therewith for preventing rotation thereof so as to effect a linear movement along said threaded shaft when said shaft is driven by said motor.
 5. The apparatus as defined by claim 3 and additionally including selsyn motor means coupled to said threaded shaft for providing an electrical readout of the mechanical setting of said variable capacitor. 